The present invention relates to cooling systems, particularly for electric arc furnaces. It will be evident from the description, however, that many principles of the invention are applicable to other systems having substantial cooling requirements, such as induction furnaces, electro slag remelting furnaces, continuous strip casting procedures, and the like.
In the production of steel by way of electric arc furnaces, a charge of scrap steel is loaded into a melting vessel through an open top. The vessel is then closed by replacement of a removable roof, and large electrodes are inserted into the mass of scrap steel. When the electrodes are energized, the scrap charge is melted.
When an adequate volume of molten metal has been developed within the vessel, it is tapped, typically by tilting of the vessel, to remove the molten metal charge. Thereafter, the furnace roof is swung aside to allow a new charge of scrap metal to be loaded. Additional scrap metal charges typically will be added during the melting process, during which, of course, the roof is swung aside to provide charging access by way of an overhead crane.
Because of the periodic removal of the roof, and also because of the "batch" character of the operation, both the roof and the furnace vessel are subjected not only to extremely high temperatures, but also to extremes of temperature cycling. As a means of reducing the high levels of temperature stress on the components of the furnace, it has become increasingly common to utilize active water cooling of the roof and walls of the furnace. Particularly the roof, and the portion of the vessel walls above the level of the molten metal are often constructed without the use of manufactured refractories, relying upon effective water cooling and a minor amount of insulation resulting from slag accumulation on the exposed surfaces. One widely used system for the water cooling of electric arc furnace walls is represented by the Zangs U.S. Pat. No. 4,207,060, and known commercially as the Demag TW-2000 System.
More recently, efforts have been made to reduce the water flow requirements of furnace cooling systems, by utilizing spray cooling procedures, such as have been used in the past in connection with the cooling of blast furnaces, for example. Such spray cooling systems have also been proposed for electro slag remelting vessels, induction furnaces and smelting cupolas, among others.
One form of spray cooling system proposed for electric arc furnaces in particular is reflected in West German Patent Publication No. 3,027,465, granted to Korf Stahl A.G. In the system of the Korf patent, the spray coolant is intended to be mostly converted to steam. The steam coolant is continuously evacuated from the vessel walls, condensed and reused. Condensate is also collected, presumably by gravity, and recycled. A similar system is illustrated in the Heggart et al. U.S. Pat. No. 4,715,042, granted to Union Carbide Corporation. In the Union Carbide system, the spray cooled vessel walls are continuously evacuated by means of venturi exhaust pumps, in order to prevent the buildup of steam pressure and/or excessive condensate within the vessel walls.
The system of the present invention provides for the spray cooling of vessel walls, particularly the roof and sidewall areas of an electric arc steel-making furnace or the like, which incorporates significant improvements over the systems proposed by the Korf and/or Union Carbide patents. In one important respect, the system of the invention incorporates significant improvements in the manner in which used spray coolant is disposed of. In the system of the present invention, the flow of spray coolant onto the vessel walls, is carried out at a relatively high rate of flow, so as, to a great extent, to avoid generation of steam. Provisions are made for the collection and disposal of the used coolant, preferably although not critically by gravity flow from the vessel walls. Significant to the invention, however, is the provision of motor-driven, disc-type pumps, which are operated substantially continuously during normal operations of the furnace and serve to remove the collected used coolant, either for recycling or for disposal, depending upon environmental and other conditions. The use of motor-driven disc-type pumps is significant, because such pumps can be operated continuously, regardless of the presence or absence of a supply of coolant. In general, mechanical pumps such as positive displacement pumps or conventional centrifugal pumps used for this purpose would quickly disintegrate if allowed to run in a "dry" condition.
By way of contrast, in the Union Carbide patent, a venturi evacuation pump system is employed, which can operate continuously without ill effect from the absence of coolant at its intake. However, the continuous operation of such pumps is extremely costly in terms of energy useage. And while the Union Carbide patent represents that the venturi evacuation system utilizes for its operation water which is otherwise available in the plant, that does not reveal the fact that such water must be substantially pressurized, up to perhaps 60 psi, in order to be effectively activate the venturi pumps. The energy required for such pressurization is extremely costly in comparison to, for example, the system of the applicant's invention, even though, technically speaking, the supply of water may be otherwise "available" for the using.
In accordance with a further feature of the invention, advantageous structural provisions are made for the continuous effective removal of used spray coolant during the forward and rearward tilting orientations of the furnace vessel, during deslagging operations and during tapping of the molten steel product. In this respect, at one or more times during a given batch operation, the furnace vessel may be tilted to the rear to enable removal of excessive slag from the surface of the molten metal. During this tilting operation, collected used coolant will tend to flow by gravity to the lowest point of the confined chamber. In the case of a spray cooled roof, such coolant will collect at the low side of the roof. Subsequently, when the molten metal batch is tapped off, the furnace vessel is tilted in the opposite or forward direction, and the coolant, which continues to be sprayed during these operations, collects at the forward side of the roof.
In accordance with one specific feature of the invention, a spray cooled roof is constructed with an internal duct extending from the front to the back sides, and connected to a pair of separate discharge outlets at one side, preferably the forward side. The discharge outlets, isolated from each other, communicate with opposite ends of the internal duct, so that in any tilted position of the vessel, used coolant can be removed.
In another advantageous form of the invention, discharge conduits, in the form of flexible hoses, are connected separately to the front and back sides of the roof and/or vessel. In any tilted orientation of the vessel, one of the discharge ducts will be connected to the low side for effective continuous outflow of the used coolant.
In an electric arc furnace, provisions typically are made for swinging aside of the roof, to provide overhead access to the vessel for charging with an overhead crane. The swinging axis of the roof typically is a vertical axis offset to one side of the furnace vessel. Particularly with respect to the last mentioned embodiment of the invention, the invention further provides for the quick-disconnect coupling of at least one of the discharge hoses, to allow the roof to be swung away from the vessel. The coupling advantageously incorporates a fluid actuator arranged, when the furnace roof is in its closed position to move the discharge hose into coupled relation with a discharge outlet on the roof. A normally closed valve member, mounted in the discharge outlet of the roof is automatically opened upon joining of the quick-disconnect coupling means.
In accordance with still another aspect of the invention, a simplified and advantageous arrangement is provided for cooling of the so-called delta section of a roof, utilizing forced circulation cooling of the delta in conjunction with spray cooling of the main body of the roof. To advantage, the exhaust coolant, from the forced circulation delta cooling system, is routed into one or more of the discharge outlets for the roof, from where it flows by gravity to the sump area.
For a more complete understanding of the above and other features and advantages of the invention, reference should be made to the following detailed description of preferred embodiments of the invention and to the accompanying drawings.